The purpose of the proposed study is to investigate the nonlinear mechanical behavior of cortical bone from the perspective of a damage accumulation process. There is considerable evidence that the nonlinear behavior of cortical bone is largely due to damage accumulation. However, in practice, bone is typically modeled as a traditional engineering material, at most as an anisotropic, elastic material, with strength described by a multi- axial strength criterion. If failure can result from damage accumulation, the use of simple properties and fixed strength values can lead to incorrect results and conclusions in stress analyses of bony structures. The application's broad hypothesis is that damage accumulation measures can predict the nonlinear stress-strain history of bone under general loading, up to failure. In this study, the applicants will experimentally measure nonlinear behavior of cortical bone in three simple loading modes: axial tension, axial compression, and torsion. They propose to investigate the predictability of failure using observable mechanical measures of damage accumulation such as maximum nonlinear strain and secant modulus degradation. They will seek to apply continuum damage models to describe the experimental behavior, and examine the ability of internal state variables to predict failure. They will further seek to characterize the domain of the induced damage by the use of local strain mapping and atomic force microscopy. The proposed basic engineering study of bone mechanics is intended to make significant contributions to the state of knowledge in several ways. First, the mechanical testing results should help define bone strength in terms of acceptable damage accumulation rates, rather than less conservative values derived from static strength tests. Second, the combined mechanical measurements, modeling effort and imaging studies provide a first step toward a more complete characterization of nonlinear behavior and multi-axial failure processes in the context of bone as a damaging material. The full accomplishment of this goal will require a great deal of work, and the proposed study will help define the amount of work and the methods required to accomplish the task. Third, characterizing the behavior of bone in terms of damage has relevance to coupling of mechanics and biology. The ability to predict damage accumulation provides the potential for predicting safe levels of repetitive loading in which repair processes can maintain bone integrity, which would be the appropriate way to describe strength of bony structures in daily living.